1. Field of the Invention
This invention relates to an index gradient type or gradient index single lens suitable as the pick-up objective of an optical disk, a collimator lens for a semiconductor laser, or the like.
2. Related Background Art
As is well known, there are gradient index type single lenses of the types which are provided with three basic index distributions, i.e., the radial type, the axial type and the spherical type. The radial type is a type in which the index distribution exists in a radial direction perpendicular to the optic axis, the axial type is a type in which the index distribution exists in the direction of the optic axis, and in contrast, the spherical type is a type in which the index distribution is sphere-symmetrical about a point on the optic axis and the surfaces of equal refractive index are concentric spherical surfaces.
In recent years, numerous propositions have been made to use a gradient index type lens as the pick-up objective of an optical disk, but most of these propositions are concerned with the gradient index type lenses of the radial type and the axial type. On the other hand, concerning the gradient index type lens of the spherical type, for example, Japanese Laid-Open Patent Application No. 181516/1982 is proposed, but this is of a special construction in which a core having the spherical type index distribution is clad with a homogeneous medium, and cannot be said to be preferable from the viewpoint of ease of manufacture. Also, attempts to use a spherical lens having the spherical type index distribution for an optical coupler or the like have been made. However, in the pick-up objective, it is necessary to correct both spherical aberration and sine condition well, and such aberration correction has been difficult in the spherical lens as described above.